Winger Rafael Leão opened the scoring for the home team with a header in the 59th minute at Dragon Stadium in Porto.
Captain Cristiano Ronaldo doubled Portugal's lead with a penalty in the 72nd minute, followed by Bruno Fernandes making it 3-0 eight minutes later.
Pedro Neto capitalised on a defence lapse in the 83rd minute to extend the score to 4-0, and Ronaldo added a fifth with a stunning overhead kick in the 87th minute.
Poland managed a late consolation goal through substitute Dominik Marczuk, who found the net in the 88th minute.
The Polish side was without their star striker Robert Lewandowski, sidelined by injury.
Captained by Inter Milan midfielder Piotr Zieliński, the Poles failed to deliver a strong performance against their higher-ranked opponents in an away fixture.
Poland are ranked 31st worldwide by football's governing body FIFA, while Portugal are No. 7 in the FIFA table.
Poland will take on Scotland in Warsaw on Monday in their final Nations League group-stage fixture.
The Nations League is the European football governing body UEFA’s newest national-team competition that is now in its fourth edition.
Portugal won the first edition of the Nations League in 2019, while France won the 2020/2021 UEFA Nations League, and Spain triumphed in the 2022/2023 edition.
Poland manager Michał Probierz has said that this season's Nations League offers a valuable opportunity to assess players and prepare them for 2026 World Cup qualifiers.
Poland boss Michał Probierz. Photo: PAP/Leszek Szymański
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Source: IAR, PAP, TVP Sport